Gravel packing is used in wells to control the production of sand and other fines from a surrounding formation. In oil and gas wells, gravel packs have served as an effective way to control the production of these particulates. Gravel is placed in a wellbore around screens or slotted liners, and the screens or liners are sized such that the gravel cannot pass through. A gravel slurry is pumped downhole into an annular region between the wellbore wall and the screen which blocks gravel from moving to the interior of the screen. The slurry carrier fluid, on the other hand, readily passes through the screen and into an open end of an internal wash pipe to be returned up through the wellbore. The gravel particles are sized to prevent sand and other fines from traveling through the gravel pack and entering the screens while allowing formation fluids to freely flow through the gravel pack and into the screens for production.
A problem common to gravel packing horizontal wells is a sudden rise in pressure within the wellbore. During gravel packing, an initial wave of gravel, the “alpha wave”, flows to the far end or “toe” of the wellbore. A return wave or “beta wave” carries gravel back up the wellbore from the toe and fills the upper portion of the wellbore left unfilled by the alpha wave. As the beta wave progresses up the wellbore, the pressure in the wellbore increases due to frictional resistance to the flow of carrier fluid. The part of the carrier fluid which is not lost to the formation by leak-off into the formation must flow back to the toe region through the small annular space between the screen and the wash pipe. At the toe region, the return flow of carrier fluid finally enters the open end of the wash pipe. Accordingly, the further the beta wave progresses, the further the carrier fluid must travel to reach the toe region. The increasing distance creates an increasing frictional resistance to the return fluid flow, causing the wellbore pressure to rise.
The increased wellbore pressure can lead to early termination of the gravel pack operation by increasing the risk that the wellbore pressure will rise above the formation fracture pressure. Such increased wellbore pressures can fracture the formation and lead to a bridge at the fracture and thus a poor quality gravel pack. Accordingly, the gravel pack operations typically are terminated before the wellbore pressure approaches formation fracture pressure, or the gravel pack procedures are designed such that the formation fracture pressure will only be reached when the beta wave has carried the gravel pack up through the wellbore over the entire screen region. This, of course, limits the length of the screen region that can be gravel packed in one time.
Attempts have been made to reduce the pressure build up during propagation of the beta wave. For example, valves have been placed along the wash pipe with the intent that the valves will open when wellbore pressure builds to effectively short-circuit or shorten the flow path of the returning carrier fluid. However, existing systems can suffer from lack of immediate or accurate control over the opening of the valves. For example, some systems are actuated from the surface via pressure pulses, which can be undesirably slow in initiating actuation of the valves. Other systems actuate the valves based on threshold pressures, rates of change in pressure or differential pressures. However, relying on threshold pressures requires use of a relatively small pressure window and incurs the risk of valves not opening in the proper sequence. Similarly, relying on rates of pressure change or differential pressures can lead to inadvertent actuation of the valves due to a variety of downhole events other than pressure increases created by the beta wave.